Active three-tone filter (CB20808E)

This circuit was found in an American documentation and can be added to common amplifiers of any type. The transistor can be any general purpose.

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Precision Limiter (CB15883E)
The circuit in the figure is based on an LM101A; diodes can be replaced with equivalents. The...
Transistorized tachometer (CB9355E)
This circuit, with magnetic sensor, uses an operational amplifier of the time. The circuit is from...
Remote control analyzer (CB7812E)
Found in a 90s Popular Electronics, this circuit checks if a remote-control transmitter is...